1. Field of the Invention
The present invention relates to code division multiple access (CDMA) spread spectrum communications. More particularly, it relates to a communications system and method which exhibits not only robustness against multipath fading, but also allows overlaid narrowband interference suppression without use of a notch filter or a RAKE tapped delay line. These advantages are achieved by a multicarrier system with direct sequence spread spectrum modulation.
2. Description of the Related Art
A typical prior art spread spectrum system is shown in FIG. 1. Message data d(t) are processed by spread spectrum data modulator 51 using a spreading-sequence signal g.sub.1 (t) to generate a spread-data signal. The spread-data signal is processed by a spread spectrum transmitter 52 using a carrier signal at a carrier frequency f.sub.o, and transmitted using suitable techniques as a spread spectrum signal over a communications channel 53. As used herein, the term "spreading-sequence signal" refers to a signal g.sub.1 (t) having a sequence of chips, which is used for spreading the spectrum of the data signal d(t). As is well-known in the art, the chip rate of the spreading-sequence signal is the rate at which the bits of data of signal d(t) are chopped into chips. Typically, the spreading-sequence signal g.sub.1 (t) includes a pseudonoise (PN) sequence. The term "spread spectrum signal" refers to a spread-data signal modulated at the carrier frequency.
At a receiver, a spread spectrum demodulator 54 despreads the received spread spectrum signal to generate a modulated data signal, and the message data are recovered by a synchronous data demodulator 60 as received data. The synchronous data demodulator 60 uses a reference signal for synchronously demodulating the despread spread spectrum signal. A square-law device 55, bandpass filter 56 and frequency divider 57, which are well-known in the art, generate a reference signal from the received modulated data signal. A Costas Loop or other reference signal generating circuit can also provide a reference signal.
A spread spectrum system has desirable characteristics such as multiple access capability, robustness against multipath fading, and antijamming capability. In addition, various techniques have been proposed to further enhance system performance over and above that offered by the processing gain associated with a spread spectrum system. For example, a RAKE receiver shows enhanced performance in the presence of frequency selective fading, and a notch filter can be used to reject strong narrowband interference.
Spread spectrum modulation allows code division multiplex (CDM) and code division multiple access (CDMA) by permitting a plurality of users to transmit and receive on the same carrier frequency f.sub.o over communications channel 53. In CDMA, each user is provided with an individual and distinctive spreading sequence for generating a spreading-sequence signal. The spreading sequences preferably are orthogonal, and typically are almost uncorrelated with one another. Since each user transmits on the same carrier frequency, the spectrum from each user overlaps. While spread spectrum modulation allows efficient use of the spectrum, a need exists for a system and method which is even more robust against multipath fading than previously existing techniques while allowing interference suppression so that the CDMA system can overlay conventional narrowband waveforms.